1. Field of the Invention
This invention pertains generally to the field of treating solid hazardous wastes, and particularly to the chemical treatment of those solid wastes containing unacceptable levels of leachable metals such as lead and cadmium.
2. Description of the Prior Art
Disposal of a hazardous waste containing excessive leachable heavy metals such as lead or cadmium can be expensive. In addition to the cost of the paperwork associated with a hazardous waste, tipping fees usually are more than $100.00 per ton, plus the cost of transportation. For example, one ferrous foundry currently has about 500,000 tons of EP Toxic waste to dispose of. Obviously, disposal of the waste as a hazardous waste is very expensive. Therefore a treatment of the hazardous waste which would render it nonhazardous could result in a tremendous cost savings for the foundry.
Currently, solid wastes are tested using the EP (Extraction Procedure) Toxicity Test contained in 40 C.F.R. Chapter 1, Part 261, Appendix II (1987), the disclosure of which is hereby incorporated by reference. The EP Toxicity Test determines whether a solid waste has unacceptable levels of hazardous substances which can be leached by infiltrating water. The test is designed to simulate a worst case leaching situation. A liquid extract is prepared from a solid waste sample, and is analyzed to determine whether the waste is "EP Toxic". A new test, the Toxic Characteristics Leaching Procedure (TCLP) has been developed and proposed by the Environmental Protection Agency (EPA), but has not yet been adopted.
In the past, foundry wastes often have been treated by the addition of magnesium hydroxide, which currently has a cost of around $20.00 per ton of waste treated. While such a process is significantly less expensive than disposal of the waste as a hazardous waste, it is still quite expensive. The chemical cost of the magnesium hydroxide for treating the 500,000 tons of sludge is approximately $10,000,000.00. The effectiveness of magnesium hydroxide in treating foundry wastes might not be simply due to the formation of metallic hydroxides and metallic carbonates, but also may be due to the magnesium hydroxide raising the pH of the waste to the point where heavy metal sorption by the iron oxides becomes significant. Thus, without the presence of iron oxides, the treatment with magnesium hydroxide might be less effective. In fact, in some applications magnesium hydroxide has been found to be somewhat ineffective in preventing the leaching of heavy metals such as lead and cadmium from solid waste.
A variety of other methods have been utilized to prevent the leaching of heavy metals in solid waste so that the waste is rendered suitable for disposal in a sanitary landfill. U.S. Pat. No. 4,701,219 to Bonee discloses stabilizing metals such as vanadium and nickel in a petroleum cracking process particulate waste matter against the leaching of the metals by rain and ground waters by mixing the waste particulate matter with a treating agent selected from the group consisting of lime, calcium fluoride, and calcium hydroxide, in an amount from about 1% to 12% by weight based on the weight of the waste particulate matter. The patent to Bonee teaches that calcium chloride, calcium carbonate, and sodium bicarbonate are relatively ineffective at reducing the leaching of heavy metals from the particulate waste matter. U.S. Pat. No. 4,268,188 to Bertus, et al. discloses the immobilization of metal contaminants in petroleum conversion or cracking catalysts by treating those contaminants with tin or indium or compounds thereof. The immobilization of heavy metals by treating an alkaline slurry of waste with a mixture of bentonite clay and Portland cement is disclosed in U.S. Pat. No. 4,149,968 to Kupiec, et al. Other patents which teach the immobilization of heavy metals in industrial wastes to make the waste suitable for disposal in a sanitary landfill include U.S. Pat. No. 4,142,912 to Young and U.S. Pat. No. 3,837,872 to Connor.
A method and apparatus for the removal of harmful and waste materials by combustion which involves the combustion of refuse with a calcium containing carbonate carrier at a temperature of at least 1200.degree. C. is disclosed in U.S. Pat. No. 4,640,681 to Steinbiss, et al. U.S. Pat. No. 4,652,381 to Inglis discloses a process of treating industrial wastewater, which has a pH of 2, and which is contaminated with environmentally unacceptable amounts of sulphuric acid and metals such as lead, copper or zinc. Calcium carbonate is added along with an oxidation medium such as air to the wastewater which is being treated. Calcium sulphate and respective heavy metal carbonates then precipitate and settle to the bottom of the treatment zone where they can be readily removed from the wastewater. Since the calcium carbonate is added to a solution with a pH of 2, a large amount of calcium sulphate is precipitated. Although the lead, copper and zinc are removed from the wastewater, the resulting sludge precipitate itself might include significant quantities of metals which could be leachable. This solid waste sludge precipitate somehow should be rendered nonhazardous before its disposal.
The patent to Inglis also teaches mixing a crystalline, normally non-reactive form of calcium carbonate such as limestone into the highly acidic wastewater. The solid waste in many foundries and metal operations often has a pH of 6-9. It has been found that limestone is relatively ineffective at removing heavy metals such as lead and cadmium from hazardous solid or sludge waste such as that which is formed in foundries and other factories which process metal. The limestone does not react in the solid waste at a high enough rate to release sufficient carbonates to react and combine with the heavy metals. Another problem with the lead-containing solid waste is that lead is amphoteric and therefore soluble at a high pH.